(1) Field of the Invention
The present invention relates to an acoustic and environmental monitoring system for studying the sound produced by wind, breaking wave occurrence and rainfall.
(2) Description of the Prior Art and of Certain Related
Concurrently Developed Technology
The traditional torpedo-launching submarine has historically operated relatively independently of sea surface conditions. Although high sea states can cause severe rolling when the keelless submarine attains periscope depth, sea surface conditions do not particularly interfere with torpedo launch at deeper depths. With the advent of submarine-launched missiles, the state-of-the-sea surface and the wind has become a much more relevant environmental parameter which can adversely impact upon these newer submarine operations. High sea states are a concern when the submarine requires near-surface launch, because any severe rolling can limit or prevent missile deployment. The problems which arise are those associated with manipulating, loading and possible launcher jamming of these large weapon systems.
Missiles, depending on the type, can be vulnerable to the effect of high sea and wind conditions immediately after exiting launcher tubes as they ascend at various speeds and exit through the sea surface. The Harpoon missile for example has suffered fatal damage to control surfaces by dynamic pressures associated with the orbital motions of larger waves. The Tomahawk cruise missile can also suffer launch failure as a direct result of inadequate knowledge of wind conditions. This missile, during its in-water, rapid, rocket-driven ascension, is relatively unaffected by wave dynamics, but upon exiting the sea surface, it becomes a light aircraft, requiring lift determined by its relative air speed. An inadvertent downwind launch in a greater than 20 m/s (40-knot) wind speed could mean loss of critical lift and a sudden, unplanned return to the sea.
Thus, it is important to establish a viable method for submarines to monitor sea conditions in order to construct computer models which predict wind speed and rainfall rate at a particular location over the ocean by measuring the acoustic signatures and to develop an acoustic and environmental monitoring system to support the development of such a viable method. One such method entails acoustic sensing of sea surface conditions. Wind, wave whitecaps and precipitation are important sources of the ambient sound in the ocean. Each of these phenomenon has a unique acoustic signature which can be recorded and used as a basis to develop models to predict sea surface conditions.
Establishing a viable submarine monitoring capability requires a clear understanding of the sound spectrum and intensity levels produced by the various sea surface phenomena. The sources of sound in the ocean are both natural and man-made, displaying large variations in frequency, geographic location, and time. The ambient sound spectrum can be conveniently divided into three bands. In each, the acoustic energy tends to be supplied by a dominant source with overlap at the edges. The low band from 10 to 400 Hz is mostly associated with machinery of shipping or small boat traffic and coastal industry. The mid band from 400 Hz to 40 kHz is contributed principally by oceanographic/meteorological (i.e., geophysical) phenomena at the sea surface. The high band above 40 kHz is mostly low level energy associated with high wave number turbulence and molecular motions. To a lesser extent, in certain local areas, there is a contribution into the mid and high bands by fish noises and cavitation from high speed propellers.
The focus is on the mid range of frequencies for the monitoring of sea surface conditions. The sound spectrum in this mid band (400 Hz to 40 kHz) appears to be produced by the following phenomena:
a) interactions of wind pressure fluctuations at the sea surface; PA1 b) splash impact and bubble formation associated with whitecapping; and PA1 c) rain, hail and snow impacting the sea surface. PA1 U is the observed wind speed, PA1 R is the observed rainfall rate, PA1 N.sub.s1 is the recorded sound pressure level (in dB) obtained from the hydrophones, and PA1 A and B are quasi constants depending on the sound source, range of frequencies and the range of intensities for either U or R.
Each of the above phenomena are by themselves complex, dynamical processes, largely turbulent in nature. Portraying the associated sound-producing mechanisms requires precise measurements and analysis to sort out the effects.
Studies of geophysical ambient sound have attempted to obtain relationships between the sound and wind speed, whitecapping (i.e., bubble formation) and rain rate. Results, however, are largely empirical and far from quantitative due to the inaccurate data obtained from the hydrophones. The general empirical relation is given as: EQU log.sub.10 U, log.sub.10 R=a(N.sub.s1)+B (1)
where
The value A may vary with wind/wave conditions, e.g., at wind speeds below 6-7 meters per second, wind pressure fluctuations should be the prime sound source; at higher speeds, multiple contributions occur from whitecap splashing and bubbles, hence increased slope is expected.
In general, the wind and wave sound falls off with frequency at about 6 dB per octave, but the rolloff is not necessarily monatomic because specific-measurement spectral peaks are seen when whitecaps form. Rain sound falls off with frequency, but displays a distinct resonant peak around 15 kHz, owing to a droplet/bubble formation at impact. Wind shows the strongest correlation with sound, showing records from 4.3-14.5 kHZ. Correlation coefficients of the sound pressure levels with logwind speed ranged from 0.92-0.96. Rain, on the other hand, generates the highest/loudest signals in its normal ranges (attaining up to 40-60 dB for heavy downpour).
Assessment of the true correlation of the ambient sound with its sources (i.e., wind speed, waves and rainfall) requires precise measurement of the surface phenomena made at the immediate location of the sound observations. A very significant obstacle to progress in this regard has been the lack of a suitable system for obtaining field measurements relating sound to surface effects with the full range of intensities of wind and wave conditions.
A wide variety of devices have been used to monitor and measure sea conditions such as wave height, wave direction and ocean currents. U.S. Pat. Nos. 3,336,800 to Appleby, Jr. et al.; 3,375,715 to Hodges et al.; 3,765,236 to Erdely; 3,769,838 to Buckler; 3,899,668 to Tucker, Jr.; 3,983,750 to Kirkland; 4,515,013 to Hue; and 4,988,885 to Lindstrom illustrate various devices for measuring waves.
The Appleby, Jr. et al. patent discloses a submarine-based system for measuring wave height and direction. The system employs continuous upward echo ranging to obtain distance measurements from the submarine to the sea surface to provide an indication of wave height and direction. The Hodges et al. patent uses a similar system where acoustic signals are directed toward the sea surface from below the sea surface. The return acoustic signals are used to indicate the slope of the sea surface above the transducers generating the acoustic signals.
The Erdley patent relates to an apparatus for measuring swell frequency and propagation direction of an ocean wave. The apparatus incudes a water gauge which is partially submerged and a ballast. A flexible connection is provided to an anchoring device. The collector encloses a magnetic compass and a counter.
The Buckler patent illustrates yet another device for determining wave height. The device includes a buoy having an antenna which when floating on the sea, its motion causes electronic circuitry within it to transmit bursts of RF signals, the pulse repetition rate varying directly in proportion to the acceleration applied to the buoy as it moves up and down on the surface of the sea. Ship-borne apparatus is provided to receive the RF signals and analyze them to determine wave height. The Tucker patent also relates to a device for electronically analyzing waves.
The Kirkland patent illustrates a water-wave height and fluid measuring system having a partially submerged support mast which floats on the water. A radio frequency transmitter is mounted on the mast and a plurality of radio frequency receivers are mounted on the mast below the water line. Signals are communicated to remote locations with output signals being analog signals which represent wave height or other fluid level.
The Hue patent relates to a buoy having accelerometers and magnetometers for measuring characteristics of an ocean swell.
The Lindstrom patent relates to a submerged small angle field-of-view optical radiometer which passively measures ocean surface wave heights, characteristics, and statistics remotely. The radiometer measures varying underwater radiant light field and correlates that light field to surface wave heights. The device is connected to on shore or inboard electronics. The sensors can be bottom mounted on the sea floor or can be mounted on a submerged platform such as a submarine.
U.S. Pat. Nos. 4,172,255 to Barrick et al.; 4,221,128 to Lawson et al.; and 4,996,533 to May et al. relate to devices for measuring and/or monitoring ocean currents. The Barrick device uses radar to remotely sense near surface ocean currents in coastal regions. In operation, the radar detects slight velocity changes in ocean waves. The Lawson patent describes an acoustic current meter in which two channels have acoustic paths oriented at right angles to each other to measure orthogonal components of a current velocity. The May patent relates to a device for mapping ocean currents with a single radar.
Still other devices illustrate systems for measuring environmental conditions such as wind and rainfall. U.S. Pat. No. 4,143,547 to Balser and U.S. Pat. No. 5,125,268 to Caron illustrate such devices. U.S. Pat. No. 3,455,159 to Gies illustrates a nautical weather station which includes at least one floating buoy, a weighted cable attached to each buoy, and one or more hollow submerged bathymetric vehicles attached at various depths to the cable. Both the bathymetric vehicles and each buoy are equipped with weather and other environmental sensors. The weather data is transmitted from each buoy to remote receiving stations on interrogation.
Still other devices for measuring environmental and oceanographic data are shown in U.S. Pat. Nos. 4,760,743 to Clifford et al.; 3,936,895 to Talkington; 5,303,207 to Brady et al. and 4,805,160 to Ishii et al.
The lack of a suitable system for obtaining field measurements relating sound to surface effects has produced empirical relations (such as equation 1) devoid of physics. The lack of accurate field measurements has been due in part to the fact that the sound measurements taken to correlate with surface effects are made at relatively large distances, of often several kilometers, from the bottom-mounted hydrophones. The acoustic signal is thus obtained at a point in the ocean relatively distant from the surface area of generation, which itself is ill defined. Because of this "far-field geometry", individual surface effects are spatially smoothed out, i.e., such measurements cannot spatially resolve noise radiated from small-scale phenomena such as breaking wave crests and turbulent wind gusts. As a result, little can be inferred about the physical sound-generating mechanism at the surface or the geometry of the generating area.
Another very significant obstacle to such progress has been that most acoustic time series data obtained from the hydrophones have been heavily smoothed over periods of at least one to several hours. As with the deep hydrophone records, this smoothing can suppress/filter or mask possibly important smaller-scale fluctuations of sound sources associated with wind gusts, breaking waves or passages of wind squalls or rain cells. Moreover, the analysis of the time variability of the ambient sound field associated with the sea surface effects has perhaps been neglected in favor of establishing smooth predictive relationships from averaged data and hence high resolution sampling has been neglected. This results in analyses that have been relatively crude and often lacking in precise spectral characterization of acoustical signatures of the individual sound generators.
Clearly, the correlation of ambient sound with its generating sources is meaningful only to the degree that the true local source producing the sound is identified and simultaneously monitored. The problem of proximity of the sound sources from the hydrophone occurs with wind, and especially, rainfall noise correlations. To wit, due to the difficulty of measuring rainfall at sea, its measurement is often made on land many kilometers from the hydrophones. This can render the correlations at best, biased, and at worst meaningless, since the rainfall is seldom uniform over such separation distances. However, rain and sound comparison measurements have been made with hydrophones at 5-35 meter depths with rain locally measured, but these were in shallow lakes, thereby placing bottom-mounted hydrophones close to the source of the noise generating mechanisms.
Thus, more appropriate data is needed to better understand the physical mechanisms of the generation of ambient sound at the sea surface. A prior art apparatus for doing this is an inexpensive, totally self contained, stand alone unit, which constitutes an easily deployed ambient sound-recording system for taking rapidly sampled near-surface measurements of ambient sound. More particularly, it takes measurements of sound associated with rapidly changing wind and rainfall events measured in close proximity. This is disclosed in a paper by David Shonting and Foster Middleton entitled "Near-Surface Observations of Wind and Rain-Generated Sound Using the SCANR: An Autonomous Acoustic Recorder" in the Journal of Atmospheric and Oceanic Technology, Vol. 5, No. 2, April, 1988.
The totally self contained, stand alone unit disclosed in the Shonting and Middleton paper is an acoustic monitoring system used by oceanographic personnel of the Department of the Navy, which will sometimes hereinafter be referred to as the First Generation, Self Contained Ambient Noise Recorder Unit 10, (or simply "SCANR-I unit 10"), which is shown in FIGS. 1, 2 and 3. FIG. 1 depicts the externally visible components of SCANR-I unit 10, which incorporates a low-noise hydrophone 12 configured for suspension from a surface buoy. The hydrophone 10, enclosed in a neoprene boot, is attached to a stainless steel support bridle 14 and mounted upright with its cable 16 leading to a pressure case 18, which in turn houses recording electronics and a battery pack. The pressure case 18 is connected to the hydrophone cable 16 by a 4-pin, water-tight connector 20.
The hydrophone employed in SCANR-I unit 10 has a free-field voltage sensitivity of -174 db//1v/uPa over the temperature range of 3.degree.-20.degree. C., remaining very flat from 0.1 to 35 kHz. The directivity response in the plane about the hydrophone's longitudinal axis over 15-22 kHz was omnidirectional to within 0.5 dB; hence, its longitudinal axis was mounted vertically to provide horizontal receiving symmetry. The hydrophone and pressure case were capable of withstanding static pressures up to 700 meter depth (70 atm).
The signal processing (including recording) components of SCANR-I unit 10, shown in FIG. 3, depict the output from a hydrophone preamplifier 22 delivered to three bandpass filters 23, 24 and 25, centered at 15, 20, and 25 kHz respectively, with the 20 kHz ban convertible to a broad bandpass from 5-40 kHz. The frequencies were chosen because they occur in bands of efficient wind, wave and rain noise generation, as well as being above most ship traffic and coastal industrial noise frequencies.
The output of each filter 23, 24, and 25 was passed through an ac/rms converter chip 26, 27, and 28 with an 800 ms integration time. The voltages were sampled at 1 minute intervals, digitized as 10-bit words (i.e., 1/1024 resolution) and recorded on a Memodyne digital cassette deck 30. The SCANR-I unit 10 was powered by 24 D-cells which sustained the three channels at a 1 minute sampling rate for 7-8 days. Upon retrieval of the unit 10, data from the Memodyne cassette 30 is transferred to a portable computer and stored on disk for subsequent processing and analysis.
A mooring arrangement for suspending A SCANR-I unit 10 at mid-depths on the continental shelf from a moored buoy is depicted in FIG. 2. Unit 10 is suspended from a standard strobe float 32 via a 5-100 meter stainless steel cable 34. This permits the contained hydrophone 12 to receive a near-surface acoustic energy isotopically from the upper hemisphere. A 20 meter buoyant polypropylene line 36 enables the strobe float 32, a second tethered wave or meteorological buoy 38, and a third float 40 to remain afloat at a desired distance. The buoy 38 shown in FIG. 2 is an Endeco direction wave buoy 956 which provides a complete set of wave height, wave direction and ambient sound data at a single location. However, this buoy 38 can be eliminated if so desired. The strobe float 32 and buoy 38 are maintained in a desired location through the attachment of the third float 40 to a 2 cm diameter nylon 2-1 scope line 42 and a Danforth chain 44. The third float 40 prevents the SCANR-I unit 10 from becoming tangled around the nylon scope line 42 by positioning the line 42 at a distance from unit 10. All shackles connecting the vertical stainless steel tether cable are taped so as to minimize surface noise as the surface float bobs up and down in the wave field. As an alternative to this mooring arrangement, the SCANR-I unit 10 can be suspended from a fixed platform or a free-drifting surface float.
Investigations of correlation between natural surface phenomena and underwater ambient sound were conducted over the period November 1984 and May 1986 in Narragansett Bay, Rhode Island during which both wind and rainfall data were obtained. A SCANR-I unit 10 was suspended at 3-4 m depths from the end of 75 m long pier which extended north-westward from a sloping beach on Aquidneck Island. This point of observation had a depth of 12 m and maximum wind fetches of 12-15 km from the northwest and 6-7 km from the southwest. Wind and rainfall were monitored simultaneously with the ambient noise recordings at narrow bands centered at 15 and 25 kHz and a broadband. Wind speed and direction were directly recorded continuously with an R. M. Young Aerovane, while rainfall rate was measured with an AES tipping bucket system on a rooftop tower located 2.2 km south of the observation pier. The wind data was obtained on a strip chart recorder while the rainfall rate was logged on a period processor and digitally recorded on a Compaq computer disk memory.
A correlation of 0.97 of the 15 kHz band with wind-speed was obtained for a 24 hour period with winds ranging from 0.2-15 m/s. At higher sustained winds (12-15 m/s), a pronounced decrease occurred in sound pressure, which decrease appears due to increased absorption of the sound generated at the surfaces by the whitecap-produced bubble layer. Use of direct sound pressure output, in lieu of logarithmic sound levels, best showed the immediate acoustic response to changes of the wind field and rainfall rates associated with passing squalls. On the other hand, the correlation coefficient between the 15 kHz band and rain rate was only 0.068. The rain-noise correlations were rendered imprecise, in part, due to the horizontal separation of the rain gauge from the hydrophone (over 2 km) and, in part, because of the differences in the sampling of the rain gauge (at a point location) vis-a-vis the SCANR hydrophone collecting acoustic energy over a large area. Rain-produced sound attained 35 dB increase within 2-3 minutes during a passing line squall, which was tracked with an MIT weather radar at Cambridge, Massachusetts.
The SCANR-I unit 10 system proved useful for observing the near-surface ambient sound field at both broad and narrow bands up to 30 kHz. It is easily deployed and retrieved while nearby observations are made of wind speed, whitecapping intensity and rainfall rate.
An improved self contained, stand alone acoustic monitoring unit was made. Its development was concurrent with the development of the present invention. Also a part of the development of the improved stand alone unit and a part of the natural surface phenomena and ambient underwater sound correlation system of the present invention consist of one-in-the-same electronic components. The concurrently developed, improved self-contained, stand alone unit will sometimes hereinafter be referred to as the Second Generation, Self Contained Ambient Noise Recorder Unit (or simply "SCANR-II"). The entire SCANR-II unit is not shown in any of the figures of drawing hereof. As will be described in the Description of the Preferred Embodiment section below, the SCANR-II processor 124, FIG. 4 constitutes the components which are common to the two developments. In external appearance the SCANR-II unit is essentially the same as the SCANR-I unit 10 depicted in FIG. 10. A hydrophone is supported by a bridle assembly, which is attached to an instrument pressure case. The pressure case is connected to the hydrophone cable by a 4-pin, water tight connector, and it contains the battery pack and the signal processing (including recording) electronics. The SCANR-II unit incorporates an ITC model 6050C hydrophone. The hydrophone, pointing upward, receives acoustic energy approximately isotopically from the upper hemisphere. The hydrophone can be used at depths to 500 meters (50 atm).
The components common to the SCANR-II unit and the present invention, namely SCANR-II processor 124, FIG. 4, provide the signal processing (including recording) function in the SCANR-II unit. Within the SCANR-II unit, the components of SCANR-II processor 124 are mounted on two MUPAC wire-wrap boards. Referring again to FIG. 4, SCANR -II processor 124 operates on a 4 MHz clock which is programmed into AN EPROM chip 84 in the conventional manner of emulation of a counter. The incoming signal from the hydrophone is amplified and passed through a 2nd order Butterworth bandpass filter 66 with -3 dB cutoffs from 0.91 to 16.2 kHz. The output is then buffered and split, with two channels running parallel through a 15 kHz narrow band notch filter 68 and a 25 kHz narrow band notch filter 70. The outputs of the narrow band notch filters 68 and 70, along with a third direct unfiltered signal 72, run into RMS chips 74 and finally into a multi-channel 8-bit A/D converter 76.
The timing and control Central Processing Unit (CPU) is based on a Z-80 microprocessor 78, manufactured by the Ziolog Company, which interfaces to external memory chips forming a nonvolatile data storage static ram 80, the above described multi-channel A/D converter 76, a serial input/output chip 82, forming a serial communications interface three Erasable Programmable Read Only Memory (EPROM) chips 84 and the battery 86 to make up a microcontroller. All ICS are Compatible Metal Oxide Semiconductor (CMOS) technology to minimize power consumption.
The improved SCANR narrow band filters 68 and 70 and wideband filter 66 were calibrated as follows: a fixed input voltage of 20.3 (peak voltage) sine wave was applied at 2 to 3 kHz increments from 0 to 35 kHz and to each of the 15 kHz and 25 kHz narrow band filters 68 and 70 and to the wideband filter 66, which produced the output voltages shown in FIGS. 5a-5c. The widths of the -3 dB down points gave bandwidths of 2.5 kHz, 4.5 kHz and 15.55 kHz for the 15 kHz, 25 kHz and wideband filters, respectively. The respective gains which were provided in each of the three effective channels, i.e. (i) input to hydrophone to output of the 15 kHz notch-type narrow band filter channel, (ii) input to hydrophone to output of the 25 kHz notch-type narrow band filter channel, and (iii) input to hydrophone to output of the 0.91 to 16.2 kHz wideband Butterworth filter; were 490, 488 and 228. These gains were effectively determined by the relative gain factor inherent to the filters, which were a matter of choice.
Referring to FIG. 4, the microprocessor 78 receives operational instructions from the three 64 Kl Erasable Programmable Read Only Memory (EPROM) chips 84; one EPROM for data acquisitions and two EPROMS for terminal communication. Data can be stored on either an 8K.times.8-bit (64K) or a 32K.times.8-bit (256K) Static Random Access Memory (SRAM) 80, which contains a lithium battery to retain data in the memory chip whenever the supply voltage falls below the operational threshold, is turned off, or if the chip is removed from the board.
An auxiliary microprocessor-based "RAM dump board", which resides in the microprocessor 78, is used to offload the data by the RS-232 interface 88 from the memory chip 80 to a laptop PC 90 for subsequent analysis. Data, downloaded in the laboratory ICs, and needed solely for outside communications, are removed to minimize power consumption.
In the event of a system hangup, there is a hardware reset switch connected to the microprocessor 78 that will restart the program. The data RAM 80 is addressed through a series of counters which are incremented by the microprocessor 78 so that if a system reset occurs, the data RAM 80 address does not get reset and no data is overwritten. The data is written in four byte blocks; first is an identification number and the rest are 8-bit values of the 15 kHz, 25 kHz and wideband RMS data. When the highest desired value is reached, the counters are reset to zero and held there so that no further data taking occurs. The counters can be reset only by a toggle switch located on the board.
Power is provided to the entire system by two battery packs 86. The first pack consists of eleven Ni-Cad 7.2 volt rechargeable individual units configured to provide three independent supplies of +14, -14 and 21.6 volts for use by the analog Board and hydrophone. All eleven individual batteries are configured in parallel and can be charged as one group. The second battery pack consists of twenty-eight Ni-Cad 1.2 volt individual units configured to provide 17 volts to a dc-to-dc converter, which in turn provides a steady regulated 5 volt supply to the auxiliary microprocessor-based "RAM dump board". Both battery packs 86 can be charged as a group by use of voltage dividers in line with the charging electrodes.
The procedure for data logging is as follows: the system is turned on and everything is reset. After an elapsed time, determined by the programmed chip, the CPU that resides in the microprocessor 78 turns on the relays to power the analog board and the hydrophone. A 10 second delay allows the RMS circuits 74 to fully energize and stabilize. The digitized values from the A/D converter 76 are read in by the microprocessor 78 and the relays are turned off. The microprocessor 78 then transfers the data to the data RAM 80, confirms the transfer, then counts down to the next sampling time. The process repeats until the last data RAM 80 address is filled. Circuitry prevents the relays from being turned on once the data RAM 80 has been filled. The sampling time is arbitrarily selected. A one minute interval was determined to be an appropriate sampling time since it was capable of capturing the pertinent meteorological events of windspeed, breaking wave occurrence and rainfall rate.
It is to be understood in use of SCANR-II unit, it is deployed in the same manner in which SCANR-I unit 10 is deployed beneath the surface of the ocean (as shown in FIG. 2 and discussed above).
Upon retrieval to a land facility of the SCANR-II unit, the digital data is extracted by means of the RS-232 serial communications interface 82 connected to the microprocessor 78. An example of the use of the SCANR-II unit is the gathering of acoustic information for comparison to environmental data obtained by above-the-ocean-surface instrumentation to provide empirical tables for estimating windspeed and rainfall rate. Once offloaded, the digitized RMS 74 data is converted to sound pressure levels which can then be plotted and compared to environmental data.
Despite the existence of the SCANR-II unit, there remains a need for a facility, particularly a shore based laboratory, for direct monitoring of the ambient sound produced by phenomena such as wind, waves and rainfall and for the study of the interrelation with environmental measurement of same.